Paris: Andre Agassi revealed on Friday he is happy to work without pay as Novak Djokovic's coach and said he would be open to extending his relationship with the Serb at Wimbledon next month.

Eight-time Grand Slam champion Agassi teamed up with Djokovic ahead of the French Open, where the Serb is attempting to defend his title.

Agassi, who won Roland Garros in 1999, said he is not being paid to mentor Djokovic but simply wants to help him find his best form again. "For me, I do this on my own time and my own dime," said Agassi. "I don't want money, I want to help him. And it helps the game. Him at his best is good for the game and it's a way I can contribute."

Agassi initially committed to just a "few matches" with the former world No. 1, but the American told Eurosport he was open to working with Djokovic at Wimbledon.

"If he wants me there, yeah, I will come," said the 1992 Wimbledon champion, speaking to Djokovic's former coach Boris Becker. "It's a lot of responsibility so whatever's practical and achievable - 100 per cent I will make the effort."

Becker gave up coaching Djokovic last December after three successful years together, while the 12-time major winner split with long-time coach Marian Vajda last month.

He then named Agassi as his coach, after he lost to Alexander Zverev in the Rome Masters final a fortnight ago.

Agassi, 47, won eight Grand Slam titles before retiring in 2006, and has since committed most of his time to his charitable foundation in Las Vegas.

He was lured back to the tennis circuit by a phone call in April from Djokovic, whose form has deserted him since he won his 12th Grand Slam title in Paris last year.

Speaking to Becker - who was among the coaching team Djokovic split from in December - Agassi revealed it was his wife, 22-time Grand Slam champion Steffi Graf, who persuaded him to try out the role.

"I said: 'Listen, maybe I can help you over the phone, I don't think you need much, but this is not possible for me with the balance of my life'," said Agassi. "Then Steffi said 'maybe you'll enjoy it.' I said, 'you think?' I respect her so much I said I'll go early, as I have to be in Paris anyway, and just get to know him."

Meanwhile, Djokovic survived his first stern test at the French Open on Friday, as he came from two sets to one down to overcome world No. 41 Diego Schwartzman 5-7, 6-3, 3-6, 6-1, 6-1 and move into the last 16. The second seed, now joint third on the all-time list for most wins at Roland Garros with 58, broke the Argentine to go 3-1 up in the first set.

But instead of cruising through he was broken twice with Schwartzman, in top form on clay this season and snatching the set as dark clouds gathered above the Court Philippe Chatrier.

The Serb recovered to secure the second set but with 42 unforced errors in the first three sets alone, including a backhand that flew long to hand Schwartzman the third, he struggled for consistency.

Under threat of becoming the first defending champion to lose prior to the round of 16 since Juan Carlos Ferrero in 2004, Djokovic kept his cool, despite a time violation and a warning, to take the match to a decider.

Two more breaks put him in control and a third gave him a spot in the last 16. (Agencies)